REACTIVE CODOPING OF GaAlInP COMPOUND SEMICONDUCTORS

ABSTRACT

A GaAlInP compound semiconductor and a method of producing a GaAlInP compound semiconductor are provided. The apparatus and method comprises a GaAs crystal substrate in a metal organic vapor deposition reactor. Al, Ga, In vapors are prepared by thermally decomposing organometallic compounds. P vapors are prepared by thermally decomposing phosphine gas, Zn vapors are prepared by thermally decomposing an organometallic group IIA or IIB compound. Group VIB vapors are prepared by thermally decomposing a gaseous compound of group VIB. The Al, Ga, In, P, group II, and group VIB vapors grow a GaAlInP crystal doped with group IIA or IIB and group VIB elements on the substrate wherein the group IIA or IIB and group VIB vapors produce a codoped GaAlInP compound semiconductor with a group IIA or IIB element serving as a p-type dopant having low group II atomic diffusion.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims priority as a divisional of U.S. patentapplication Ser. No. 10/552,102, filed on Oct. 3, 2005 and entitled“Reactive Codoping of GaAlInP Compound Semiconductors” by Mark E. Hannaand Robert Reedy, hereby incorporated by reference as if fully set forthherein.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. DE-AC36-99GO10337 between the United States Department of Energy andthe National Renewal Energy Laboratory, a division of the MidwestResearch Institute.

BACKGROUND OF THE INVENTION

This invention relates generally to a GaAlInP compound semiconductor andthe production of GaAlInP compound semiconductors and, moreparticularly, it relates to reactive codoping of GaAlInP compoundsemiconductors for improved electrical conductivity and dopant stabilityin heavily doped layers.

DESCRIPTION OF THE PRIOR ART

Semiconductor devices, such as solar cells and LEDs, require regionswith differing types (n-type or p-type) and levels of conductivity.Adding impurity atoms, known as dopants, either to add or remove freeelectrons, varies the conductivity of the semiconductor device. For somesemiconductor materials, it is difficult to obtain high p-typeconductivity because of the solubility, activation, and diffusiondynamics of the materials in question. Heavy p-type doping of GaAlInPcompound semiconductors with high Al content, such as AlInP, isdifficult to achieve with single acceptor species such as Zn, Mg, or Be.Therefore, a method for production of GaAlInP compound semiconductors isneeded which greatly enhances the incorporation and solubility ofacceptor impurities in GaAlInP grown lattice matched to GaAs substrateswhich is useful with conventional techniques such as metal organicchemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). It isalso desirable that the method produces low resistivity p-type materialwith the important benefit of reduced atomic diffusion of the acceptordopants.

SUMMARY

The present invention is a method of producing a GaAlInP compoundsemiconductor. The method comprising the steps of disposing a GaAssingle crystal as a substrate in a metal organic chemical vapordeposition (MOCVD) reactor, preparing Al, Ga, In vapors by thermallydecomposing organometallic compounds of Al, Ga, and In, preparing Znvapors by thermally decomposing an organometallic Zn compound, preparingP vapors by thermally decomposing phosphine gas, simultaneouslysupplying the Al, Ga, In, P, Se, and Zn vapors to a region for epitaxialcrystal growth on the substrate, and epitaxially growing a GaAlInPcrystal doped with Zn and Se on the substrate wherein the Zn and Sevapors supplied to the region for epitaxial crystal growth produce acodoped GaAlInP compound semiconductor with Zn serving as a p-typedopant at an atomic ratio of Zn:Se greater than two (2) in the GaAlInPcrystal.

The present invention additionally further includes a method ofproducing a GaAlInP compound semiconductor. The method comprises thesteps of disposing a GaAs single crystal as a substrate in a metalorganic vapor deposition reactor, preparing Al, Ga, In vapors bythermally decomposing organometallic compounds of Al, Ga, and In,preparing P vapors by thermally decomposing phosphine gas, preparinggroup II element vapors by thermally decomposing an organometallic groupIIA or IIB compound, preparing group VIB vapors by thermally decomposinga gaseous compound of group VIB, simultaneously supplying the Al, Ga,In, P, group II, and group VIB vapors to a region for epitaxial crystalgrowth on the substrate, and epitaxially growing a GaAlInP crystal dopedwith group IIA or IIB and group VIB elements on said substrate whereinthe group IIA or IIB and group VIB vapors supplied to the region forepitaxial crystal growth produce a codoped GaAlInP compoundsemiconductor with a group IIA or IIB element serving as a p-type dopanthaving low group II atomic diffusion at an atomic ratio of II:VI greaterthan approximately two (2) in the GaAlInP crystal.

The present invention further includes a GaAlInP compound semiconductor.The GaAlInP compound semiconductor comprises a substrate with thesubstrate being a GaAs single crystal positioned within a metal organicchemical vapor deposition (MOCVD) reactor. Al, Ga, In vapors arethermally decomposed from organometallic compounds of Al, Ga, and In. Pvapors are thermally decomposed phosphine gas. Group II element vaporsare thermally decomposed from an organometallic group IIA or IIBcompound. Group VIB vapors are thermally decomposed from a gaseouscompound of group VIB with the Al, Ga, In, P, group II, and group VIBvapors being applied to a region for epitaxial crystal growth on thesubstrate wherein the group IIA or IIB and group VIB produce a codopedGaAlInP compound semiconductor with a group IIA or IIB element servingas a p-type dopant having low group II atomic diffusion at an atomicratio of II:VI greater than approximately two (2) in the GaAlInPcrystal.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the preferred embodiments of the presentinvention, and together with the descriptions serve to explain theprinciples of the invention.

In the Drawings:

FIG. 1 is a schematic view of a metal organic chemical vapor deposition(MOCVD) reactor, constructed in accordance with the present invention;

FIG. 2 is a graph of a secondary ion mass spectroscopy (SIMS) profile ofthe atomic Zn concentration in an AlInP crystal doped with Zn only andthe measured electrical properties;

FIG. 3 is a graph of a secondary ion mass spectroscopy (SIMS) profile ofthe atomic Zn and Se concentrations in an AlInP crystal codoped with Znand Se and the measured electrical properties illustrating that lowresistivity p-type AlInP was obtained;

FIG. 4 is a graph of a secondary ion mass spectroscopy (SIMS) profile ofthe atomic Zn concentration in a highly Zn-doped AlInP crystal dopedwith Zn only illustrating significant Zn diffusion from AlInP layer intothe GaAs substrate; and

FIG. 5 is a graph of a secondary ion mass spectroscopy (SIMS) profile ofthe atomic Zn concentration in a highly Zn-doped AlInP crystal codopedwith Zn and Se illustrating greatly reduced Zn diffusion compared toFIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is a method utilizing two (2) doping atoms, ann-type (Se) and a p-type (Zn), to increase the p-type conductivitythereby greatly enhancing the incorporation of Zn acceptor impurities inAlInP lattice-matched to GaAs compound substrates. The method of thepresent invention uses the simultaneous doping with both donor (Se) andacceptor (Zn) impurities to enhance the solubility of Zn and the dopinglevel in p-type AlInP. By increasing the n-type donor to a sufficientlevel, the incorporation of the p-type acceptor will increasesignificantly thereby lowering the resistivity and increasing theconductivity of the AlInP crystal.

As illustrated in FIG. 1, the AlInP epitaxial layers were grown on GaAssubstrates by low-pressure metal organic chemical vapor deposition(MOCVD). It should be noted that while the present invention describesusing low pressure metal organic chemical vapor deposition (MOCVD),other techniques of growing the AlInP epitaxial layers are within thescope of the present invention.

As illustrated in FIG. 1, the low pressure metal organic chemical vapordeposition (MOCVD), indicated generally at 10, feeds purified H₂ and N₂carrier gas 12 into a computer controlled gas switching manifold 14. Thecarrier gas 12 flows through the gas switching manifold 14 wherein thecarrier gas 12 is saturated with hydride gaseous sources 16 (AsH₃, PH₃,H₂Se, and Si₂H₆, for example) and organometallic liquid sources 18(trimethylaluminum (TMAl), triethylgallium (TEGa), trimethylindium(TMIn), and dimethylzinc (DMZn), for example). The saturated gas 20 isthen transported into a quartz reactor 22, where the saturated gas 20impinges on a single substrate crystal 24 of GaAs resting on a heatedSiC susceptor 26. Preferably, the quartz reactor 22 has ahydrogen/arsine gas ambient with the GaAs substrate crystal 24 heated toa temperature of approximately 640° C. by radio frequency conduction orother means.

By controlling the amounts of Ga, Al, In, the composition of the layerdeposited on the GaAs substrate crystal 24 can be determined. In thepresent embodiment, a 0.1 micron thick GaAs layer is deposited on theGaAs substrate crystal 24 at approximately 640° C. by introducingtriethylgallium into the reactor for a controlled amount of time, i.e.,approximately ten (10) minutes to produce an atomically flat surfacewith monolayer high steps and terraces. The GaAs substrate and epilayerare cooled to approximately 600° C. in the hydrogen/arsine gas ambient.

The gases phosphine, trimethylindium, trimethylaluminum, dimethylzinc,and hydrogen selenide are then transported into the quartz reactor 22 todeposit a p-type layer of AlInP codoped with zinc and seleniumimpurities. The incorporation of zinc is increased by a factor ofapproximately four (4) to approximately ten (10) over a layer with zincdoping only. This leads to a highly conductive p-type crystal with astable Zn-dopant profile, i.e., Zn atomic diffusion in the Zn and Secodoped layer is greatly reduced compared to the a Zn-doped layer only.

A vacuum pump 28 can be connected to the quartz reactor 22 to reduce thepressure within the quartz reactor 22 and a hydride scrubber 30 can beconnected to the quartz reactor 22 to remove any residual gases whichare not decomposed.

The AlInP epitaxial layers were simultaneously doped with Zn from DMZnand Se (from H₂Se). Up to a six-fold increase in the atomic Znconcentrations up to 8×10²⁰ cm⁻³ were obtained. Reduced Zn outdiffusionfrom codoped layers with high Zn concentrations were also obtained.Codoped AlInP with high hole concentrations of up to 6×10¹⁷ cm⁻³ withcorresponding low resistivities of 0.26 ohm-cm were obtained. Thiscodoping method of the present invention can also use otherdonor-acceptor pairs such as Mg or Be acceptors atoms and S or Te donoratoms.

EXAMPLES

The following examples illustrate the group II acceptor dopingenhancement by codoping AlInP with a group IIA acceptor and a group VIBdonor. In these examples, Zn is used as the group II acceptor and Se isused as the group VI donor although other group II acceptors and othergroup VI donors are within the scope of the present invention.

Example 1

A GaAs single crystal with a 511A crystallographic orientation was usedas a substrate for the growth of an AlInP single crystal and was locatedin a metal organic chemical vapor deposition (MOCVD) reactor held atseventy-five (75) torr. AlInP single crystals closely lattice matched tothe GaAs were epitaxially grown at temperatures of 540° C.-600° C. bysupplying 0.4 sccm of trimethylaluminum, 0.22 sccm of trimethylindium,and 250 sccm of phosphine in ten (10) slm of hydrogen carrier gas. Zndoping was accomplished by introducing dimethylzinc at flow rates of0.32 to 3.2 sccm. Se doping was accomplished by introducing a gasmixture consisting of 100 ppm of H₂Se in hydrogen at flow rates of two(2) to one hundred (100) sccm. Various combinations of Zn doped only andZn+Se codoped AlInP crystals were grown. The atomic Zn and Seconcentrations, N_(Zn) and N_(Se), respectively, were measured bysecondary ion mass spectroscopy (SIMS) and are summarized in Table 1. Inthe AlInP crystal grown at 600° C., the Zn doping level was increasedfrom 4.5×10¹⁸ cm⁻³ in the zinc doped only crystals (samples 1-3) to1.1-3.0×10¹⁹ cm⁻³ in Zn+Se codoped crystals (samples 4-6). In the AlInPcrystal grown at 540° C., the Zn doping level was increased from 2×10²⁰cm⁻³ in the zinc doped only crystals (sample 7) to 8×10²⁰ cm⁻³ in theZn+Se codoped crystals (sample 8).

DMZn Temperature flowrate H₂Se flowrate N_(Se) N_(Zn) Sample (° C.)(sccm) (sccm) (cm⁻³) (cm⁻³) 1 600 0.32 0 0 4.5 × 10¹⁸ 2 600 1.6 0 0 4.5× 10¹⁸ 3 600 3.2 0 0 4.5 × 10¹⁸ 4 600 0.32 2 2.8 × 10¹⁸ 1.1 × 10¹⁹ 5 6000.32 5 6.5 × 10¹⁸ 1.7 × 10¹⁹ 6 600 0.32 10 1.1 × 10¹⁹   3 × 10¹⁹ 7 5400.32 0 0   2 × 10²⁰ 8 540 0.32 100 2.5 × 10²⁰   8 × 10²⁰

Example 2

A GaAs single crystal with a 511A crystallographic orientation was usedas a substrate for the growth of an AlInP single crystal and was locatedin a metal organic chemical vapor deposition (MOCVD) reactor at heldseventy-five (75) torr. Zn doped only and Zn+Se codoped AlInP singlecrystals closely lattice matched to the GaAs were epitaxially grown onan undoped AlInP buffer layer at temperature of 600° C. by supplying 0.4sccm of trimethylaluminum, 0.22 sccm of trimethylindium, and 250 sccm ofphosphine in ten (10) slm of hydrogen carrier gas. Zn doping wasaccomplished by introducing dimethylzinc at flow rate of 1.6 sccm. Sedoping was accomplished by introducing a gas mixture consisting of 100ppm of H₂Se in hydrogen at a flow rate of ten (10) sccm. The profiles ofatomic Zn and Se concentrations, N_(Zn) and N_(Se), respectively, in Zndoped only and Zn+Se codoped AlInP single crystals were measured bysecondary ion mass spectroscopy (SIMS) and are illustrated in FIGS. 2and 3, respectively. The hole concentration, p, the mobility, μ, and thehole resistivity, ρ, were measured by the Hall effect and areillustrated in FIGS. 2 and 3. In the zinc doped AlInP crystal, the Zndoping level was N_(Zn)=3.5×10¹⁸ cm⁻³, p=6.2×10¹⁷ cm⁻³, μ=62 Vs/cm², andρ=0.16 Ωcm. In the Zn+Se codoped AlInP crystal, the Zn doping levelincreased to N_(Zn)=2×10¹⁹ cm⁻³, while the hole concentration, mobility,and resistivity were p=5×10¹⁷ cm⁻³, μ=48 Vs/cm², ρ=0.26 Ωcm. Theseresults demonstrated that low p-type resistivity was obtained in Zn+Secodoped AlInP.

Example 3

A GaAs single crystal with a 511A crystallographic orientation was usedas a substrate for the growth of an AlInP single crystal and was locatedin a metal organic chemical vapor deposition (MOCVD) reactor at heldseventy-five (75) torr. Very heavily Zn doped AlInP single crystalsclosely lattice matched to the GaAs were epitaxially grown attemperature of 540° C. by supplying 0.4 sccm of trimethylaluminum, 0.22sccm of trimethylindium, and 400 sccm of phosphine in ten (10) slm ofhydrogen carrier gas. Zn doping was accomplished by introducingdimethylzinc at flow rate of 0.32 sccm. Se doping was accomplished byintroducing a gas mixture consisting of 100 ppm of H₂Se in hydrogen at aflow rate of one hundred (100) sccm. The profiles of atomic Zn and Seconcentrations, N_(Zn) and N_(Se), respectively, in Zn doped only andZn+Se codoped AlInP single crystals were measured by secondary ion massspectroscopy (SIMS) and are illustrated in FIG. 4 and FIG. 5,respectively. At these high Zn concentrations, significant Zn diffusionfrom the AlInP crystal into the GaAs substrate crystal occurs as shownin FIG. 4 in the Zn-doped only AlInP crystal. An diffusion is greatlyreduced in the codoped AlInP crystal as illustrated in FIG. 5, resultingin a very sharp Zn dopant profile even though the Zn concentration isfour times larger that the case of FIG. 4.

The present invention uses reactive codoping to enhance incorporation ofZn acceptor impurities in AlInP lattice-matched to GaAs substrates. Thepresent invention additionally uses the simultaneous doping with bothdonor (Se) and acceptor (Zn) impurities to enhance the solubility of Znand the doping level in p-type AlInP. The present invention findsindustrial applicability in the use of GaAlInP compound semiconductorsfor: a high gap top-cell, window layer, tunnel junction, or back-surfacefield layer of tandem solar cells; red to yellow-green high brightnessvisible LEDs for energy efficient traffic signals; and visible laserdiodes for digital video disc technology. Crystal production uses themolecular beam epitaxy (MBE) process or a metal organic chemical vapordeposition (MOCVD) process.

The compound semiconductor alloy GaAlInP of the present invention haswidespread technological and commercial applications in optoelectronicdevices such as tandem solar cells, high brightness visible LEDs, and invisible laser diodes. The present invention utilizes codoping of AlInPwith Zn and Se to enhance Zn incorporation and stabilize Zn diffusion inhighly p-type dopes AlInP.

The foregoing exemplary descriptions and the illustrative preferredembodiments of the present invention have been explained in the drawingsand described in detail, with varying modifications and alternativeembodiments being taught. While the invention has been so shown,described and illustrated, it should be understood by those skilled inthe art that equivalent changes in form and detail may be made thereinwithout departing from the true spirit and scope of the invention, andthat the scope of the present invention is to be limited only to theclaims except as precluded by the prior art. Moreover, the invention asdisclosed herein, may be suitably practiced in the absence of thespecific elements which are disclosed herein.

1. A GaAlInP compound semiconductor, the GaAlInP compound semiconductorcomprising: a substrate, the substrate being a GaAs single crystalpositioned within a metal organic chemical vapor deposition (MOCVD)reactor; Al, Ga, In vapors thermally decomposed from organometalliccompounds of Al, Ga, and In; P vapors thermally decomposed phosphinegas; Group II vapors thermally decomposed from an organometallic groupIIA or IIB compound; and group VIB vapors thermally decomposed from agaseous compound of group VIB, the Al, Ga, In, P, group II, and groupVIB vapors being applied to a region for epitaxial crystal growth on thesubstrate; wherein the group IIA or IIB and group VIB produce a codopedGaAlInP compound semiconductor with a group IIA or IIB element servingas a p-type dopant having low group II atomic diffusion at an atomicratio of II:VI greater than approximately two (2) in the GaAlInPcrystal.
 2. The GaAlInP compound semiconductor of claim 1 wherein themetal organic chemical vapor deposition (MOCVD) reactor has a gasswitching manifold and the Al, Ga, In vapors are prepared in the gasswitching manifold.
 3. The GaAlInP compound semiconductor of claim 1wherein the metal organic chemical vapor deposition (MOCVD) reactor hasa gas switching manifold and the group II vapors are prepared in the gasswitching manifold.
 4. The GaAlInP compound semiconductor of claim 1wherein the metal organic chemical vapor deposition (MOCVD) reactor hasa gas switching manifold and the P vapors being prepared in the gasswitching manifold.
 5. The GaAlInP compound semiconductor of claim 1wherein the metal organic chemical vapor deposition (MOCVD) reactor hasa gas switching manifold and the group VIB vapors being prepared in thegas switching manifold.
 6. The GaAlInP compound semiconductor of claim 1wherein the metal organic chemical vapor deposition (MOCVD) reactor hasa reaction chamber and a susceptor positioned within the reactionchamber with the substrate being positioned on the susceptor.
 7. TheGaAlInP compound semiconductor of claim 6 wherein the substrate isheated to a temperature of between approximately 500° C. and 650° C. 8.The GaAlInP compound semiconductor of claim 1 wherein the sources areselected from the group consisting of AsH₃, PH₃, H₂Se, and Si₂H₆.
 9. TheGaAlInP compound semiconductor of claim 1 wherein the organometalliccompounds are organometallic sources selected from the group consistingof trimethylaluminum (TMAI), triethylgallium (TEGa), trimethylindium(TMIn), and an organometallic compound of group IIA or group IIB.